1. Field of the Invention
The present invention relates to a reducer of an electric power steering apparatus. More particularly, the present invention relates to a reducer of an electric power steering apparatus that is capable of: correctly assisting a driver's steering wheel operating force by preventing a clearance from occurring by the wear and tear of a worm and a worm wheel; reducing impact transferred from a road surface and rattle noise generated by a backlash; and minimizing a variation of the clearance according to the change of a rotational torque.
2. Description of the Prior Art
FIG. 1 is a view illustrating a configuration of a conventional electric power steering apparatus. As illustrated in FIG. 1, a conventional electric power steering apparatus 100 includes: a steering system 130 continued from a steering wheel 102 to opposite vehicle wheels 126; and an assist power mechanism 140 configured to provide a steering assist power to the steering system 130.
The steering system 130 includes a steering shaft 106 that is connected, at one side, to the steering wheel 102 to be rotated with the steering wheel 102, and connected, at the other side, to a pinion shaft 108 via a pair of universal joints 104. In addition, the pinion shaft 108 is connected to a rack bar 112 through a rack and pinion mechanism 110, and the opposite ends of the rack bar 112 are connected to the vehicle wheels 126 through tie rods 122 and knuckle arms 124, respectively.
The assist power mechanism 140 includes: a torque sensor 142 configured to sense a torque that is applied to the steering wheel 102 by the driver and to output an electric signal that is proportional to the sensed torque; an electronic control unit 144 configured to generate a control signal based on the electric signal transferred from the torque sensor 142; a motor 146 configured to generate an assist power based on the control signal transferred from the electronic control unit 144; and a reducer 150 including a worm 152 and a worm wheel 156 in order to transmit the assist power generated by the motor 146 to the steering shaft 106.
Accordingly, the electric power steering apparatus is configured such that a torque generated by the rotation of the steering wheel 102 is transmitted to the rack bar 112 via the rack and pinion mechanism 110, and the assist power generated by the motor 146 according to the generated torque is transmitted to the rack bar 112.
That is, the torque generated by the rotation of the steering wheel 102 and the assist power generated by the motor 146 are combined with each other so as to cause the rack bar 112 to move in an axial direction.
FIG. 2 is a cross-sectional view illustrating a reducer of a conventional electric power steering apparatus. As illustrated in FIG. 2, the reducer 150 is provided with a worm shaft 254 on which the worm 152 is formed, and worm shaft bearings 257 are respectively installed at the opposite ends of the worm shaft 254 to support the worm shaft 254. In order to prevent the worm shaft bearings 257 from being loosened in the axial direction of the worm shaft 254, a plug bolt 210 is fastened between a damping coupler 240 and the worm shaft bearings 257, and the plug bolt 210 is fixed by a plug nut 220.
The worm shaft 254 has a structure in which the worm shaft 254 is connected with the motor 146 via the damping coupler 240 such that the worm shaft 254 is rotated by the driving of the motor 146.
In addition, a worm wheel 156 is provided at one side of the outer diameter of the worm to be engaged with the worm 152 formed on the worm shaft 254. The worm wheel 156 is configured to transmit the torque of the worm shaft 254 applied by the driving of the motor to the steering shaft 106 by being mounted on the steering shaft 106 configured to transmit the torque of the steering wheel 102 operated by the driver.
The worm 152, the worm wheel 156, and so on are provided within a reducer housing 260, and the motor 146 is provided at one side of the reducer housing 260 to provide a driving force to the worm shaft 254. The reducer housing 260 and the motor 146 are coupled to each other by a motor cover 230 with a bolt 250.
Each of the worm shaft bearings 257 includes balls 258 interposed between an inner race 280 and an outer race 270 so as to support the rotation of the worm shaft 254 connected to the motor 150 at the end of the reducer housing 260.
The reducer of the electric power steering apparatus configured as described above is configured to control the driving of the motor by an electronic control unit provided in a vehicle depending on a running condition of the vehicle, and to cause the torque of the worm shaft applied by the driving of the motor to be transmitted to the steering shaft in addition to the torque of the steering wheel operated by the driver such that the driver's steering and driving conditions can be maintained smoothly and stably.
However, the conventional reducer of the electric power steering apparatus has a problem in that when the worm and the worm wheel, which are rotated by the driving of the motor, are aged, noise is generated due to the clearance occurring by the wear and tear of the worm and the worm wheel.
In addition, there is a problem in that noise is generated due to impact transferred from a road surface through the vehicle wheels and the steering shaft and a backlash between the worm and the worm wheel.
Accordingly, there is also a problem in that the steering assist power, which assists the driver's steering wheel operating force, cannot be correctly transmitted due to the problems described above.